1. Field of the Invention
The invention relates to an additive-agent diffusion plate structure and an additive-agent diffusion plate that diffuse an additive agent injected into an exhaust passage for an engine, at a position upstream of an exhaust gas purification device disposed in the exhaust passage.
2. Description of the Related Art
Exhaust gas discharged from an engine, particularly, a diesel engine, generally contains pollutants generated, by combustion, for example, nitrogen oxides (hereinafter, referred to as NOx) such as nitrogen monoxide. To prevent air pollution, it is strongly required to reduce the amount of such pollutants discharged from the diesel engine. NOx may be discharged, together with exhaust gas, from a direct injection gasoline engine in which gasoline is injected directly into a combustion chamber, depending on an operating condition. Therefore, it is also required to reduce the amount of the pollutants discharged from the direct injection gasoline engine.
Thus, an exhaust gas purification device that includes a three-way catalyst is provided in an exhaust passage, to purify NOx discharged together with the exhaust gas.
However, the exhaust gas purification device including the three-way catalyst may not sufficiently purify NOx, depending on the type of the engine. For example, when employing a lean-burn diesel engine, the exhaust gas contains excessive oxygen, and therefore, a fuel component (HC) easily reacts with the oxygen (i.e., the fuel component (HC) is easily combusted). Thus, it is difficult for the three-way catalyst to sufficiently purify NOx.
Accordingly, for example, Japanese Utility Model Application Publication No. 3-68516 describes a technology in which an exhaust gas purification device including a zeolitic catalyst is provided in an exhaust passage, and a fuel component (HC component) is supplied to exhaust gas, at a position upstream of the exhaust gas purification device, to efficiently purify NOx.
Also, for example, Japanese Patent Application Publication No. 2005-113688 (JP-A-2005-113688) describes a technology in which an exhaust gas purification device including a selective reduction NOx catalyst is provided in an exhaust passage, and urea is supplied to exhaust gas, at a position upstream of the exhaust gas purification device, to efficiently purify NOx in the exhaust gas.
The additive agent, such as the fuel component or the urea, needs to be efficiently diffused in the exhaust gas, to increase NOx purification performance of the exhaust gas purification device.
Thus, for example, Japanese Patent Application Publication No. 10-165769 (JP-A-10-165769) describes a technology in which a rectifying lattice, which includes a gas mixing promoter that promotes mixing of exhaust gas and an additive agent (a fuel component or a urea aqueous solution), is provided in an exhaust passage, at a position between a position from which the additive agent is supplied, and an exhaust gas purification device. The rectifying lattice stands in a direction substantially perpendicular to a direction in which the exhaust gas flows (hereinafter, referred to as “exhaust-gas flow direction”), and occupies an entire cross section of the exhaust passage. Thus, the rectifying lattice efficiently diffuses the additive agent in the exhaust gas.
Also, for example, Japanese Patent Application Publication No. 2007-32472 (JP-A-2007-32472) describes a technology in which an additive agent is supplied from an upper position (supply position) on a peripheral wall portion of an exhaust passage; and a plurality of plates are disposed at a plurality of levels from an upper level to a lower level, below the supply position. The additive agent supplied into the exhaust passage sequentially collides with the plates from the upper level so that the additive agent is diffused in the exhaust gas. In this case, the plurality of the plates include upper porous plates and a lowermost flat, plate. The additive agent supplied into the exhaust passage collides with the upper porous plates at portions other than hole portions. The lowermost flat plate receives the additive agent that has passed through the hole portions of the upper porous plates. This prevents adherence of the additive agent to a wall surface of the exhaust passage.
The above-described additive-agent diffusion plate including the rectifying lattice is disposed in the exhaust passage to stand in the direction substantially perpendicular to the exhaust-gas flow direction, and to occupy an entire cross section of the exhaust passage. That is, the additive-agent diffusion plate is disposed to extend in the direction orthogonal to the exhaust-gas flow direction. In addition, the gas mixing promoter of the additive-agent diffusion plate includes a gas swirl generator that protrudes to face the flow of the exhaust gas, and a gas agitator that is bent in an inverted V-shape to face the flow of the exhaust gas. Therefore, a flow passage area of the exhaust passage, through which the exhaust gas flows (i.e., a sectional area of the exhaust passage) is greatly decreased by the gas mixing promoter (the gas swirl generator and the gas agitator). This results in a significant increase in a back pressure downstream of the additive-agent diffusion plate in the exhaust-gas flow direction.
Further, the additive-agent diffusion plate including the rectifying lattice is formed by combining partition plates in a vertical direction and a horizontal direction. In addition, the gas mixing promoter includes the gas swirl generator that protrudes from the partition plate, and the gas agitator disposed on the partition plate to be bent in an inverted V-shape. Therefore, the additive-agent diffusion plate has an extremely complicated structure.
In the above-described additive-agent diffusion plate including the plurality of plates, although the plates are disposed at the plurality of levels, an injection range, to which the additive agent is injected, has an extremely large diameter near the lowermost flat plate, because the additive agent is injected in a conical shape whose apex is the supply position (injection port). Therefore, the additive agent that has passed through the hole portions of the porous plates may not collide with the lowermost flat plate. As a result, the performance of diffusing the additive agent may not be increased.
Further, in the additive-agent diffusion plate including the plurality of plates, the flow passage area of the exhaust passage, through which the exhaust gas flows, may be decreased by the plates, because the plurality of plates need to be provided at the plurality of levels. This makes it difficult to suppress the increase in the back pressure downstream of the additive-agent diffusion plate in the exhaust-gas flow direction.